To accommodate optically enabled MCM (multi-chip module) packages having high port counts, it is anticipated that external laser light sources will be the norm in order to alleviate thermal concerns. In CMOS (complementary metal oxide semiconductor) photonic circuits, an external source lased light is injected vertically by positioning an optical fiber at a slight angle into a PGC (polarization grating coupler), which has the effect of separating the source laser light into its TE (transverse electric) and TM (transverse magnetic) components. The PGC also rotates the TM component into a TE polarization which easily propagates through a silicon substrate in the lateral direction. Depending on the number of optical channels being driven by a single laser light source, a single CW (continuous wave) light source can be split into N “equal” intensities to be fed into N distinct MZI (Mach-Zehnder interferometer) modulators, effecting N egress optical data channels.
When building an MCM package or CSP-OE (chip scale package-optical engine) package, it is typically assumed that all optical egress channels require identical link budgets. But in a practical system this may not be the case, and some channels sourced by the same laser may need to drive extremely short/long or lossless/high-loss links simultaneously. Utilizing in-line SOAs (semiconductor optical amplifiers) or dedicated LD/VOAs (laser diode/variable optical attenuator) per channel with feedback circuitry is a costly system solution, especially for devices supporting a large channel count.
Conventional passive optical splitters integrated onto a silicon die or PLC (planar light wave circuit) cannot be altered, or reconfigured after the fabrication process of the circuit. Conventional hybrid polymer waveguide circuits, which are less costly to fabricate, are not thermally stable to integrate into an MCM package due to the high junction temperatures of semi-conductor electronics. Fabrication of custom dies having tailored optical splitter ratios (e.g. 99/1, 90/10, 80/20, etc.) rarely achieve their designed coupling ratios from one die to the next due to across wafer process variations, and small batch fabrication is an extremely expensive approach to achieve optical power management for each egress optical channel.